As a reflector or a reflection plate that receives a radio wave, a reflectarray is known such that multiple elements, which are small comparable to a wavelength, are arranged in a planar fashion. Such a reflect array is also referred to as a “metamaterial.” Each of the elements is formed by a structure that reflects the radio wave. For example, the elements may be formed in mushroom-like structures. The mushroom-like structure may include at least a ground plate and a patch that is disposed on the ground plate through an insulator. The patch and the ground plate may be connected by a via.
FIG. 1 shows a state where the multiple elements which are included in the reflectarray are arranged in an x-axis direction. Though it is not shown, the elements are similarly arranged in a y-axis direction. The elements are formed in the mushroom-like structures. In the depicted example, the radio wave is reflected in a direction such that an angle of reflection is θr. For simplicity, an incident wave is not depicted. In order to suitable reflect radio waves in a direction of a desired reflection angle θr, each of the radio waves that is reflected by the corresponding element is required to have a suitable reflection phase. In the depicted example, the four elements reflect radio waves having corresponding phases. Here, the phases are different by 90 degrees one another. In general, N elements are designed so that the neighboring elements reflect radio waves such that the reflection phases of the radio waves are different by Δφ. In this case, Δφ=(2π/λ)·Δx·sin(θr) holds. Here, λ is a wavelength of the radio wave, and Δx is element spacing. For achieving such a reflectarray, in an example of a conventional technique, data of a relationship that holds between the reflection phase and a size of the patch is obtained in advance, and the size of the patch of each of the elements is determined in accordance with the relationship. FIG. 2 schematically shows the relationship between the reflection phase and the size of the patch. The size of the patch of the element is defined in accordance with the “size of the patch” which corresponds to the “desired phase.”
For the reflectarray to reflect the radio waves in the desired direction, it is preferable that a total of the reflection phases (which is N×Δφ) of the corresponding elements among a predetermined number of elements (N elements in FIG. 1) is 360 degrees. However, as shown in FIG. 2, unachievable reflection phases exist in the vicinity of plus and minus 180 degrees. Thus, elements may not be produced which require corresponding reflection phases which are close to plus or minus 180 degrees, and it is possible that a characteristic of a reflected wave by the reflectarray is lower than the desired characteristic. From a perspective of reducing the unachievable reflection phases, it is considered to combine multiple element structures such that the height of the patch, the length of the via, or the thickness of the substrate of one of element structures is different from that of another one of the element structures, and to use the combined element structures (in this regard, refer to Patent Document 1). In this case, it may be required that electrically conducting layers are layered on the substrate. Here, the number of the electrically conducting layers corresponds to the types of the element structures to be combined. Thus, the manufacturing process is complicated, and it is possible that higher cost, in addition to effort, is required for manufacturing.
Further, when the size of the patch of the element is adjusted by using the relationship such as shown in FIG. 2, space (a gap) between large patches becomes very narrow, and such elements may not be achieved due to processing accuracy of processing the patches. In that case, it is possible that the characteristic of the reflected wave by the reflectarray is lower than the desired characteristic.
As described above, in the conventional reflectarray which utilizes the mushroom-like structure, values of the reflection phases are varied by changing a value of capacitance which is determined by the gap between the elements of the mushroom-like structures and a value of inductance which is determined by the length of the via.